In order to enhance the anti-hydroplaning performance of the tire, it is general means to increase a groove width and a groove depth of a tread groove formed in a tread surface, and to enhance the dewatering performance. According to such means, however, as the pattern rigidity is reduced and a groove volume is increased, the steering stability and the noise performance on a dry road are deteriorated.
Thereupon, in recent years, an attempt has been made to employ an inclined groove instead of a longitudinal groove which extends in a circumferential direction of the tire, and to enhance the anti-hydroplaning performance while suppressing the deterioration of the steering stability and the noise performance. With such an attempt, a tread end of the inclined groove is formed into a gently inclined groove portion having an angle close to an axial direction of the tire, thereby securing a pattern rigidity in a tread shoulder region, and a portion of the inclined groove closer to a tread equator is formed into an steep inclined groove portion having an angle closer to the circumferential direction of the tire, thereby enhancing the dewatering performance.
With such a technique, however, the anti-hydroplaning performance can not be enhanced sufficiently, and further improvement of the anti-hydroplaning performance especially at the time of turning is desired.
This is because that in the dewatering performance near the tire equator, a dewatering effect obtained by the longitudinal groove is more excellent than a dewatering effect obtained by the inclined groove. When a vehicle turns, a position where a ground pressure becomes highest is shifted toward a tread end in the grounding surface. At that time, in the case of a tire having the inclined groove, the position where the ground pressure becomes highest is shifted to a gently inclined groove portion of the inclined groove, and sufficient dewatering effect can not be exhibited when the vehicle turns.
The present invention has been accomplished in view of such circumstances, and based on an idea that the longitudinal grooves and the inclined grooves are used in a specific combination, it is an object of the invention to provide a pneumatic tire capable of largely enhancing the anti-hydroplaning performance while suppressing the deterioration of the steering stability and the noise performance.